See the Flambda manual chapter for an explanation in prose of the inlining decision procedure.

val for_call_site : env:Inline_and_simplify_aux.Env.t -> r:Inline_and_simplify_aux.Result.t -> function_decls:Simple_value_approx.function_declarations -> lhs_of_application:Variable.t -> closure_id_being_applied:Closure_id.t -> function_decl:Simple_value_approx.function_declaration -> value_set_of_closures:Simple_value_approx.value_set_of_closures -> args:Variable.t list -> args_approxs:Simple_value_approx.t list -> dbg:Debuginfo.t -> simplify:Inlining_decision_intf.simplify -> inline_requested:Lambda.inline_attribute -> specialise_requested:Lambda.specialise_attribute -> Flambda.t * Inline_and_simplify_aux.Result.t

Try to inline a full application of a known function, guided by various heuristics.

val should_inline_inside_declaration : Flambda.function_declaration -> bool

When a function declaration is encountered by for_call_site, the body may be subject to inlining immediately, thus changing the declaration. This function must return true for that to be able to happen.